The preparation of polycarbonate can be achieved through the melt reaction of an aromatic dihydroxy compound and a diaryl carbonate. There are several methods by which diaryl carbonate can be produced including decarbonylating a diaryl oxalate in the presence of a catalyst while removing a carbon monoxide by product; reacting an aromatic hydroxy compound with phosgene in the gas phase in the presence of a heterogeneous catalyst, for example, the direct phosgenation of phenol; reacting an aromatic hydroxy compound, carbon monoxide, and oxygen in the presence of a redox catalyst and an organic salt; or reacting an aromatic hydroxy compound with a dialkyl carbonate. A specific example of a non-phosgene route to synthesize the diaryl carbonate of diphenyl carbonate (DPC) can be achieved with the use of respective catalysts through the transesterification of dimethyl carbonate (DMC) and phenol to produce phenyl methyl carbonate (PMC) as shown in Reaction (1),
followed by the subsequent disproportionation of PMC to produce diphenyl carbonate (DPC) as shown in Reaction (2),
with an additional formation of small amounts of an alkyl aryl ether (anisole) as the main reaction byproduct.
The formation of diaryl carbonates in any of the aforementioned reaction schemes or in any other reaction scheme can generally be facilitated through the use of a catalyst. Unfortunately, any residual metal from said catalyst can result in discoloration of a resultant polycarbonate resulting in, for example, a reduction in the color stability of the polycarbonate. Furthermore, the metal from the catalyst used in the formation of the diaryl carbonate can cause corrosion of the reaction vessel that can result in a further source of metal corrosion, in addition to any degradation of the reaction vessel that can occur independently of the metal catalyst.
A purified diaryl carbonate reactant is therefore desirable in the production of polycarbonate for use in high transparency applications.